Heretofore, a saturated hydrocarbon, such as isobutane, has been regarded as being an inert gas. For example, in Japanese Patent Application Laid-open Specification No. 55-2619, there is a description to the effect that isobutane is used as a diluent for a reaction gas in the oxidation of an olefin or an aldehyde. Since isobutane is less reactive, as seen from the above, the conventional processes for the preparation of methacrolein or methacrylic acid from isobutane (see for example, Japanese Patent Application Laid-open Specification No. 58-189130) generally consists in the converting of isobutane to isobutylene by the use of a dehydrogenating catalyst or an oxidative dehydrogenating catalyst, and oxidizing the formed isobutylene to obtain the desired product.
In recent years, researches have been carried out with a one-step conversion process by the oxidation of isobutane to a valuable compound such as methacrolein and methacrylic acid. In British Patent No. 1340891, although the yield is extremely low, there is a description suggesting a one-step process for the production of methacrolein from isobutane by contacting a mixed gas of isobutane and oxygen in the vapor phase with an oxide comprising antimony, molybdenum, etc. However, according to the process of the British patent, methacrylic acid cannot be obtained.
It is U.S. Pat. No. 4,260,822 that first taught a one-step process for the production of methacrylic acid from isobutane. According to the process of the U.S. Patent, methacrylic acid is obtained from isobutane, with a high selectivity, in one step, using a specific catalyst comprising oxides of antimony, molybdenum and phosphorus. For example, when the concentration of isobutane is 10 mol%, 10% of the isobutane participates in the reaction, 50% of which is converted to methacrylic acid. However, under such conditions, the methacrylic acid content of the reaction gas is as low as 0.5 mol %, and accordingly, the productivity per unit weight of the catalyst is extremely poor. Moreover, when the reaction is performed at an increased isobutane content of 28 mol %, the yield is likely to decrease. Hence, the process of U.S. Pat. No. 4,260,822 is unsatisfactory as a commercial process from the viewpoint of productivity per unit weight of the catalyst.
Thereafter, in Japanese Patent Application Laid-Open Specification No. 62-132832, a process for preparing methacrylic acid in high yield and with high productivity was proposed, which comprises reacting isobutane in a concentration as high as 30 to 60 mol %, using a heteropoly acid as a catalyst. High yield and high productivity have been realized by this process, which has extremely characteristic features in that a heteropoly acid having phosphorus as a central element and containing molybdenum is used as the catalyst, and that isobutane and oxygen are alternately contacted with the catalyst. However, a special reaction apparatus is necessary for alternately contacting isobutane and oxygen with the catalyst, and the operation of such an apparatus is complicated. Therefore, the process of the Japanese Patent Application is likely to be economically disadvantageous from the viewpoint of a commercial practice, as compared to the conventional process which comprises simply contacting a mixed gas of isobutane and oxygen with a catalyst.
Moreover, it is known that with respect to a molybdenum heteropoly acid, the structure thereof is decomposed at a temperature higher than 350.degree. C. although the rate of the decomposition is low. Despite this fact, reaction in the above-process is carried out at a temperature of from 350.degree. to 370.degree. C. This is because the activity of the catalyst is insufficient and hence a low temperature reaction causes the yield to be gravely lowered. Actually, decomposition of a heteropoly acid occurs even at about 330.degree. C. in the above reaction. Therefore, a catalyst capable of providing excellent yield even at a temperature of 320.degree. C. or lower has been desired in the art.
More specifically, with respect to a process for preparing methacrylic acid by contacting a mixed gas of isobutane and oxygen in the vapor phase with a catalyst, an improved process which can be carried out at a temperature of 350.degree. C. or lower, preferably 320.degree. C. or lower to produce methacrylic acid with high productivity and in high yield while maintaining a stable activity of the catalyst over a prolonged period of time, has been strongly desired.